Scientist suspects big shake tied to ‘bending’ Pacific Plate

Yesterday’s 6.2 earthquake was likely the effect of contorting deep within one of the two tectonic plates New Zealand straddles.

GeoNet recorded it about 25km southwest of Taumarunui — but what mattered more was its 207km depth.

Professor John Townend of Victoria University of Wellington suspected the earthquake was produced by bending within the Pacific Plate, which is constantly sliding and grinding against the Australian Plate.

As the Pacific Plate effectively dived beneath the North Island, the further west in the island the deeper the plate was.

But quakes that struck at the depth of yesterday’s did still send seismic waves propagating to shallower parts of the great slab — which meant it would have been felt more strongly in Hawke’s Bay and Wellington.

Townend described the Pacific Plate slab as a “funnel” that sent seismic waves across a wider area when a quake happened. “[it] is old, cold and rigid, and transmits seismic waves very efficiently.”

He said the quake would have resulted either from the two plates rubbing together or from the Pacific Plate contorting, which was likelier.

The quake’s physical orientation — or “focal mechanism” — suggested to Townend that it was reflecting stresses within the Pacific plate induced by a combination of flexure and thermal stresses. “The cold Pacific slab is heating up as it goes deeper and deeper into the mantle.”

The jolt’s aftershock potential wasn’t clear — but large, deep quakes like yesterday’s usually caused fewer aftershocks than shallower ones.

New Zealand’s geology could also explain why people in Auckland wouldn’t have felt the quake.

The seismic waves it generated would have lost their energy as they moved northward and hit hot mantle rock deep beneath the North Island. “It would be like walking through hot toffee — it just soaks up all the energy.”

This was also why it wasn’t felt as strongly even in Taumarunui, as the waves had to propagate directly upward, through the hot mantle.

The waves would have also been slowed by crustal rock near the North Island’s surface — and then further absorbed by Auckland’s volcanic geology.